An organic electroluminescence (EL) device is classified into a fluorescent organic EL device and a phosphorescent organic EL device, and a device design optimum for the emission mechanism of each type of organic EL devices has been studied. It is known that a highly efficient phosphorescent organic EL device cannot be obtained by merely applying a fluorescent device technology due to the emission characteristics thereof.
Specifically, since phosphorescence emission utilizes triplet excitons, a compound used for forming an emitting layer must have a large energy gap. This is because the energy gap (hereinafter often referred to as “singlet energy”) of a compound is normally larger than the triplet energy (in the invention, the triplet energy means the difference in energy between the lowest excited triplet state and the ground state) of the compound.
As the material for a phosphorescent organic EL device, Patent Document 1 discloses a phosphorescent host material having a line-symmetrical structure, for example.